When all four of the nationwide wireless carriers rewrote their unlimited-data wireless plans over the past few weeks – making them more complex and, in their cheaper forms, more limited – they didn’t really retire their older plans.

Although new customers can’t decline these new offerings from AT&T, Sprint, T-Mobile and Verizon, subscribers to their old plans can keep them, just as they could through earlier rate changes.

That’s not always a good thing – the unlimited-data plans that AT&T and Verizon once sold, now each $90 factoring in unlimited-texting-and-calling packages, cost more than their latest unlimited options. But in certain other situations, holding on to a grandfathered plan can save serious cash.

For example, if you switched to an unlimited-data plan in late February 2017, after Verizon’s return to selling uncapped data goaded its rivals into sweetening their unlimited deals, you probably have a better bargain than what you’d get today. If so, ignore your carrier’s enticements to switch.

But many users still find that limited-data plans work better –their allotments more than cover their data consumption, something you can check by consulting your bill, but they cost $20 or $30 a month less. AT&T and Verizon even sell them to new customers.

A survey of 1,176 subscribers released July by the research firm Cowen and Company found that only 56.3 percent of them were on unlimited plans. At AT&T and Verizon, both of which still offer cheaper limited plans to new customers, the figures were much lower: 53.4 percent and 47.5 percent, versus 80.2 percent of Sprint subscribers and 85 percent of T-Mobile’s.

“There have been periodic efforts by carriers to move them from legacy plans,” wrote analyst Jeffrey Moore, founder of Wave7Research, in an e-mail. But they’ve made little difference, he added. “You’d be surprised how many Verizon and AT&T customers are still on shared data plans.”

He noted one case of enthusiasts profiting especially well from a long-gone option, the “Framily” shared-usage plan Sprint launched in 2014 that let you build a group of up to seven unrelated people to max out the per-line discount on either 1 GB per line or unlimited data. Today, four years after Sprint quit selling Framily in 2014, Sprint customers still match up with other Framily members on a Reddit forum.

Moore also called out a plan that’s aged especially badly, also from Sprint: the “Simply Everything” plan it introduced in 2008, which runs $100 a month for unlimited data.

Timing when to change plans remains difficult, however. The Cowen survey offers one reminder: Respondents’ single-line bills, taxes and fees included, averaged $104 a month for iPhones and $93 for other smartphones. That strongly suggests many remain on older, pricier rates.

A mirror lends me another reminder: I’m one of those T-Mobile subscribers who didn’t jump on that $70 T-Mobile One rate when it was available.

(Disclosure: I also write for Yahoo Finance, a property of Verizon’s Oath media division.)